1. Field of the Invention
The present invention relates to a porous membrane such as an ultrafiltration membrane, a reverse osmosis membrane, a microfiltration membrane, etc., used for the purpose of filtration and separation of proteins, colloids, bacteria, viruses, salts, etc., in various separation processes in the food industry, pharmaceutical industry, and electronics industry, and in waste water treatment, artificial organs, purification of fresh water from sea water, etc., and in addition relates to the preparation of such a membrane.
2. Prior Art
As a method for preparing porous membranes, there has been a so-called wet process in which a polymer is dissolved in a solvent and a transformation procedure from a sol to a gel was utilized, as well as a method in which both a polymer compound and a material which were extractable by a nonsolvent were dissolved in a solvent suitable for both compounds, and after a film was made by evaporating the solvent, the film was extracted with the non-solvent.
However, in these methods, not only were there problems in that the production speed was slow, but also it was necessary to prepare a polymer solution, and it was impossible to obtain a porous membrane having excellent strength, heat resistance and chemical resistance.
In order to solve these problems, a membrane with a cross-linked structure was prepared. As a method for making this possible, in Japanese Patent Publications No. 34,329/1981 and No. 65,220/1988, a method was described for preparation of a porous membrane with a cross-linked structure in which a polymerizable monomer and/or oligomer were polymerized in the presence of a non-solvent which acted as a solvent for the monomer and/or oligomer, and did not swell a polymer made of these monomers. Furthermore, in Japanese Laid Open Patent No. 107,062/1974, an asymmetric membrane was described characterized by the membrane consisting of a covalently cross-linked vinyl polymer.
However, even though a porous membrane with excellent strength, heat resistance, and chemical resistance could be obtained with high production speed by the method of Japanese Patent Publications No. 34,329/1981 and No. 65,220/1988, since a porous membrane with a uniform pore diameter distribution in the cross-sectional direction of the membrane was obtained, in order to obtain a porous membrane with an arbitrary filtering separability, it was necessary to select membrane forming conditions in very narrow ranges. In addition, when filtering separation was performed by using the porous membrane, there were problems in that the permeation speed of the filtrate was extremely low and fouling of the membrane occurred easily. Furthermore, the method of Japanese Patent Publication No. 65,220/1987 was restricted to a method for preparation of a finely porous membrane with a fine pore dimension of 0.02-15 micrometers and without the capability of molecular weight cut-off.
In addition, in the Proceedings of the Fourth Annual Membrane Technology/Planning Conference 231 (1986), there was given a description of an asymmetric polymer membrane obtained by irradiating a homogeneous polymerizable solution in which a monomer and/or an oligomer were polymerizable by irradiation by an energy ray, and a non-solvent which acted as a solvent for the monomer and/or the oligomer and which did not swell or dissolve a polymer produced of the monomer and/or oligomer cured by an energy ray. However, in this reference, only a description of a gas separation membrane with a ratio of permeation of oxygen to nitrogen of 2.9-4.2 was given; that is, one having no pores which communicate with one another which could let a liquid pass through, and no description was given of a practical method for preparation thereof.
In addition, in the method of Japanese Laid Open Patent Publication No. 107,062/1974, as this asymmetric membrane was prepared by exploiting the freezing of a solvent (hereinbelow described as a "freezing method"), the thickness of a layer with a smaller pore diameter than that of the other parts in the thickness direction of the membrane (hereinbelow described as a dense layer) was 50 micrometers or greater, and this resulted in the filtering rate being very low; and the pore diameter of a layer with a larger pore diameter than that of either part in the thickness direction of the membrane, (hereinbelow described as a "porous supporting layer") and resulted in a membrane with insufficient membrane strength. This is not practical.